/**
 * 
 * ast解析
 * 
 *  const a = 5 + 5;
    const c = 5 + 5 / 4 * 10
    const d = 5 * 5 + 3 * 2
    结果：
    {"operation": "=", "arguments":["a", {"operation": "+",  "arguments": [5, 5]}]}
    {"operation":"=","arguments":["c",{"operation":"+","arguments":[5,{"operation":"*","arguments":[10,{"operation":"/","arguments":[5,4]}]}]}]}
    {"operation":"=","arguments":["d",{"operation":"+","arguments":[{"operation":"*","arguments":[5,5]},{"operation":"*","arguments":[3,2]}]}]}
 */
margin-top:50%;的含义
+ ~ 选择符的含义
